This invention relates to driver circuits with magnetic isolation for high power semiconductor devices and more particularly to gate driver circuits for gate turn-off thyristors (GTOs) and base driver circuits for bipolar transistors.
Gate turn-off thyristors (GTOs) require gate drive current of both polarities. The initial power required at the instant when the polarity switches is much higher than the steady bias power required to hold the device on or off. Many GTO converter circuits require that the gate driver operate at high variable voltage with respect to ground. The amount of gate power is such that magnetic coupling is the only practical method of power isolation at present. The rise times of the switching current pulses must be short and their timing precise, with minimum delay in response to control signals. In many converter circuits, the on or off time of the GTO may be short (50 .mu.s), so both the on and off drivers must reset quickly to be ready for switching polarity again. This fast-reset feature is also needed for protective schemes in which the GTO may be turned on or off any time in response to detection of a fault. This combination of requirements makes a design of a small, efficient and economical gate driver circuit rather difficult.
It is an object of the present invention to provide a gate driver circuit in which the number of magnetic components is limited to two transformers for each driver circuit.
It is a further object of the present invention to provide a gate driver circuit which avoids the need for any isolation component other than transformers.
It is a still further object of the present invention to provide a gate driver circuit which generates both high current initial pulses and low steady bias power with high efficiency in both polarities.
It is another object of the present invention to provide a gate driver circuit which allows short on or off time by using fast-reset circuits.
It is yet another object of the present invention to minimize the size of a gate driver circuit for a GTO, allowing it to be mounted close to the GTO device since short gate leads allow fast rise times.
It is a further object of the present invention to avoid damage to a gate driver circuit for a GTO in the event of failure of the GTO device by making operation safe with the gate-cathode terminals short-circuited or open-circuited.
It is a still further object of the present invention to provide a driver circuit for a high power bipolar transistor.